The healthcare trend to Point-of-Care devices using disposable cartridges and smaller device formats calls for a miniaturization of existing fluid based tests. The resulting need to analyze smaller fluid volumes has driven the development of micro fluidic chips to measure the properties of particles in solutions on a single particle basis.
The current state of the art principle for electrical particle analysis is the Coulter counter. Here particles in an electrolyte solution are drawn through a small aperture, separating two electrodes between which an electric current flows. The voltage applied across the aperture creates a “sensing zone”. Particles which pass through the aperture displace their own volume of electrolyte and therefore change the impedance of the aperture. This change in impedance produces a pulse which is characteristic of the size of the particle. This apparatus enables to determine the size distribution and concentration of the particles in the fluid. Coulter counters have also been realized in a microfluidic format.
Microfluidics also enable to probe the impedance in a channel in a sideways fashion. Probing the fluid by opposing or adjacent electrodes inside a channel was consequently proposed. In macrofluidics, similar structures have been proposed inside an aperture, and with multiple electrode pairs.
Cheung et al. describe in “Impedance Spectroscopy Flow Cytometry: On-Chip Label-Free Cell Differentiation”, Cytometry Part A 65A: 124-132 (2005), a microfluidic chip that uses two pairs of electrodes 10a, 11a and 10b, 11b inside a narrow microfluidic channel 12, as shown in FIG. 1. Here, while one of the electrode pairs senses the particle, the other pair acts as a reference.
Both top electrodes 11a, 11b are connected to the same AC or DC input signal (A=B) while the bottom electrodes 10a, 10b are connected to ground (signal C). The currents passing through the fluid in the micro fluidic channel 12 between the left and the right electrode pair are measured, and the corresponding impedances between the left and the right electrode pair are determined, amplified and their difference is taken by standard analog electronics. The in-phase and out-of-phase parts of the resulting AC signal are measured using standard Lock-in-technology. Without a particle passing the electrodes 10a, 11a; 10b, 11b the determined difference signal is preferably, although not necessarily, zero (in practice always an offset may be present due to electronic component inaccuracies). If a particle 13 coming from the left passes first the left electrode pair 10a, 11a a positive almost Gaussian shape like signal is produced when sensing the impedance difference between the electrode pairs. When the particle 13 afterwards passes the right electrode pair 10b, 11b a negative Gaussian shape signal is produced when sensing the impedance difference between the electrode pairs. The resulting antisymmetric double Gaussian signal shape can be seen at the bottom part of FIG. 1.
The standard way to analyse the measurement, as also described in the reference mentioned above, is to use a thresholding algorithm to find the particle events. Only the amplitude of the double Gaussian signal is used. It is determined by simply taking the difference between the maximum and the minimum of the signal curve. The time delay T between maximum and minimum can be used to determine the speed of the particle 13.